Coaxial cables are usually composed of an elongated outer tubular conductor of metal containing a concentrically situated elongated central conductor of metal, both conductors being separated by a layer of an electrically insulating material. The central conductor may be composed of a single wire or bundle or wound wires, also known as litz.
Coaxial cables are used in many areas such as transmission and computer cables, computer networking, video signal transmission, instrumentation cables, broadcast cables, e.g. TV companies between the community antenna and user homes or businesses, telephone companies, medical e.g. ultrasound devices, and lightweight coaxial cables for satellites. For some of these applications, miniature coaxial cables are desired and an upper limit as to the overall thickness of the cable is required. This is particularly important in invasive ultrasound and surgery equipments, where the cables that deliver the information travel through delicate human tissues or organs. For such applications, all components of the coaxial cable should be as thin as possible while meeting the physical as well as electrical requirements necessary for proper shielding to prevent interference and eliminate cross-talking within the pack of cables used.
With most consumer electronic products that use coaxial cables, the shielding layer is made of a braided mesh-like metal layer, that envelopes around the center conducting metal layer, or of aluminum foil, which is laid around or glued to the insulating layer. These structures are not suitable for thin wires due to technological limitations.
For the shielding of thin wires, direct coating of the conducting layer is usually used. The most common techniques employ vacuum deposition of thin aluminum film on the insulating layer, coating the insulating layer with a conductive lacquer, or electroplating a metal layer on the insulating layer. From the alternatives of electroplating metals, electrolytic copper gives the best results in terms of conductivity, solderability, flexibility and long-term environmental resistance. However, for using this technique, it is necessary to ensure good adhesion of the electroplating metal to the insulating layer.
Several attempts have been made in recent years to attend the demand for reducing the diameter of coaxial cables that are used in miniaturized measuring, information handling and information communication equipments. For example, in order to provide a coaxial cable that is strong against bending and is compact, Japanese Patent Application No. 2000-138013, published on May 16, 2000, proposes a coaxial cable with an extruded coating insulator made of fluorinated resin of at least 35μ thickness on a conductor, an electroless-plated metal layer on the insulator which surface has been treated by a surface-reforming treatment, for example, by excimer laser or chemical treatment, an electroplated metal layer on the electroless-plated metal layer, and a protective covering layer on the outer layer. In order to improve plating adhesion, Japanese Patent Application No. 2000-138014, published on May 16, 2000, proposes a coaxial cable with the same characteristics as the coaxial cable of JP 2000-138013, but further having a thin ABS resin sheath layer applied on the extruded coating fluorinated resin insulator of at least 35μ thickness that underwent aggressive excimer laser or chemical treatment, said thin ABS resin layer promoting a satisfactory adhesion between the insulator reformed surface and the plated layer. The use of extruded fluorinated resins or extruded polypropylene does not permit to obtain very thin insulator layers, as are needed in coaxial cables for certain applications, particularly for ultrasound equipment for medical purposes.